Many mold factories only judge thermocouple quality by appearance and price during procurement, lacking systematic performance detection standards, resulting in a large number of low-performance sensors entering the production line and triggering continuous plastic defective products. A complete set of detection indicators covering temperature measurement accuracy, high-temperature resistance, anti-interference, mechanical fatigue and sealing performance can effectively verify whether hot runner thermocouples meet long-term mass production standards. Each detection item corresponds to actual production working conditions, and qualified thermocouples can reduce hot runner failure rate by more than 70%.
Temperature measurement accuracy linearity detection is the core inspection item. Standard constant temperature oil bath and high-temperature heating furnace are used to simulate hot runner working temperature environment, collecting thermoelectric potential signals output by thermocouples at temperature points of 100℃, 200℃, 300℃ and 400℃ respectively, and comparing with national standard thermocouple parameter values. Qualified K-type hot runner thermocouples shall have temperature deviation within ±0.8℃ below 350℃, and N-type high-temperature models shall not exceed ±1℃ at 400℃. If the deviation of the test data exceeds the standard value, the alloy wire raw material has impure proportion, which will produce serious temperature drift after short-term production. In addition to static linearity detection, dynamic response test should be carried out: transfer the thermocouple from normal temperature environment to 300℃ heating platform, record the time required for the displayed temperature to reach the actual platform temperature, and qualified products need to complete response within 3 seconds, ensuring real-time feedback of hot runner temperature fluctuation.
High-temperature aging resistance detection simulates long-term baking working conditions. Put the thermocouple into a constant temperature oven at 380℃ for continuous 72-hour aging test, and test the temperature measurement accuracy before and after aging. The deviation change after aging shall not exceed 0.5℃; products with large deviation change have poor anti-oxidation performance of alloy wires, and will drift rapidly after being installed on hot runners for one to two months. For thermocouples matched with high-temperature engineering plastic hot runners, 450℃ ultra-high temperature aging test is required, and the sheath surface shall not have obvious oxidation discoloration or crack deformation after the test. The outer protective wire shall not have melting, embrittlement and powder falling off under long-term high temperature baking.
Electromagnetic anti-interference performance detection simulates workshop electromagnetic environment. Place the tested thermocouple beside the injection molding machine servo motor and high-power heating coil, and continuously record temperature data for 1 hour. Qualified full-shield thermocouples shall not have temperature jump exceeding 0.5℃ during the test; unshielded or single-layer shielding products will produce irregular temperature fluctuations of more than 3℃, which are easy to trigger false temperature alarm and unstable plastic melting state. Electronic mold production lines must pass this anti-interference test standard before thermocouple procurement.
Mechanical bending and compression fatigue detection verifies service life under mold assembly and disassembly. Fix the thermocouple wire end on the reciprocating bending test machine, carry out 10,000 times of small-angle bending test at the common bending position of mold assembly, and then conduct open circuit detection. Qualified products keep internal alloy wires intact without breakage. Compression test simulates the extrusion of thermocouple sheath by mold plates; after applying 500N pressure for 1 hour, the sheath shall not be deformed and cracked, and internal insulation performance remains normal. Spring compression thermocouples need additional spring fatigue test, after 5,000 times of compression rebound, the spring elastic force shall not decrease obviously to avoid loose contact of sensing head.
Sealing and anti-corrosion detection simulates plastic volatile erosion environment. Place the thermocouple in a closed container filled with plastic acid volatile gas for 48 hours, then detect internal insulation resistance. The insulation resistance of qualified products is higher than 1000MΩ; if the sealing at the wire outlet is poor, corrosive gas will invade the interior and reduce insulation resistance, easily causing short circuit fault. Sheath corrosion test adopts mixed acid solution spraying, and there shall be no perforation and rust spots on the surface after 24 hours spraying for anti-corrosion alloy sheath thermocouples.
Enterprises can take the above detection standards as procurement acceptance specifications, requiring suppliers to provide corresponding test reports for each batch of thermocouple products. Strict performance screening before warehousing can avoid a large number of hidden dangers in subsequent hot runner production.
